(i) Field of the Invention
The present invention relates to the operation of fluxing metallic surfaces which is generally involved before operations of soldering or tinning in electronics. It thus applies in particular to the fluxing involved before the operations:
of soldering components onto a circuit (both in the case of inserted components and surface-mounted components), PA1 of soldering contact strips onto electronic supports, making it possible to connect the support in question to another support (mention may be made here of the example of a hybrid circuit or of a printed circuit which is to be inserted using these contacts into a printed circuit, or a hybrid or printed circuit which can be plugged, using this connection edge, into a connector), PA1 of soldering circuits into bottoms of casings (involved during encapsulation of such circuits), PA1 of soldering involved during encapsulation-casing closure procedures, PA1 of soldering bare chips onto supports such as a printed circuit, hybrid circuits or else multilayer interconnection substrates such as the substrates commonly called MCM (multi-chip modules), PA1 of tinning circuits or terminations of electronic components. PA1 its efficiency (ratio of power input for creating the plasma to the density of species produced which actually interact with the support to be treated), or else the permissible power density (in the case of dielectric-barrier discharge, it reaches only a few W per cm.sup.2 of dielectric) which, if they were to be enhanced, might permit shorter treatment times; PA1 and also the fact of limiting "geometrical" factors: in the case of corona discharge, the electrode/sample distance is highly critical and must be kept very small, which may cause problems in the case of substrates whose surface structure is relatively convoluted; in the case of microwave discharge, it gives rise to the formation of a plasma-generating spot which has defined dimensions limited by the plasma source; PA1 moreover, a plasma as created in this document contains, by definition, ionic species and electrons (and therefore electrically charged species) which are always difficult to use on electronic components. PA1 a) an initial gas mixture comprising an inert gas and/or a reducing gas and/or an oxidizing gas is passed through at least one apparatus for forming excited or unstable gas species, in order to obtain a primary gas mixture at the outlet of the apparatus; PA1 b) the surface to be fluxed is treated, at a pressure close to atmospheric pressure, with a gaseous treatment atmosphere comprising excited or unstable species and substantially free of electrically charged species which is obtained from the primary gas mixture in question. PA1 to operate substantially at atmospheric pressure, PA1 to obtain a high degree of flexibility with regard to the distance between the object to be treated and the device used for carrying out this treatment, PA1 to avoid contact of the articles with charged species, PA1 to offer an improved power density, making it possible to achieve an enhanced treatment rate. PA1 a) at least one of the apparatuses for forming excited or unstable gas species converts a different initial gas mixture from that converted by the apparatus preceding it in the said structure, and/or PA1 b) the adjacent gas mixture employed in at least one of the apparatuses for forming excited or unstable gas species is different from that employed in the apparatus preceding it in the said structure. PA1 fluxing according to the invention (cold or hot) followed by a preheating operation (followed by soldering or tinning); PA1 a preheating operation followed by fluxing according to the invention (cold or hot) (followed by soldering or tinning); PA1 fluxing according to the invention (cold), followed by a preheating operation, followed by fluxing according to the invention (hot) (followed by soldering or tinning), PA1 this list of successions being, of course, only illustrative of the numerous possibilities afforded by the invention, and in no way limiting. PA1 the supply of primary gas mixture to each apparatus is stopped; PA1 a reduced flow rate of the primary gas mixture which circulates in the apparatus in question (for example a few % or a few tens of % of the flow rate circulating in the nominal regime used for treatment) is maintained in each apparatus; PA1 when an adjacent gas mixture is employed in at least one of the apparatuses, the supply of adjacent gas mixture is stopped in at least one of these apparatuses; PA1 instead of the primary gas mixture which circulates in the apparatus in question, a primary gas mixture which can be referred to as the "standby" is passed through each apparatus (for example a neutral gas or else a neutral gas/hydrogen mixture, etc.); PA1 each apparatus is switched from the treatment regime which it was in to a standby regime in which the power density used therein is only a few W/cm.sup.2.
The expression "tinning" should be understood to mean operations of depositing layers whose composition may be widely varied (including, but not exclusively, depositing layers of the tin/lead type).
(ii) Description of the Related Art
Two of the methods most commonly used for carrying out such soldering (or tinning) operations are called "wave soldering (or tinning") and "reflow soldering".
In the first case (wave soldering machines), the design of these machines is such that the articles to be soldered or to be tinned are brought into contact with one or more waves of liquid solder which are obtained by circulation of the solder bath contained in a vat through one or more nozzles.
In the case of the second type of method (reflow soldering), which term moreover encompasses a plurality of techniques, use is made no longer of a liquid solder bath but of a soldering paste, containing the solder alloy, which is deposited on the support (for example an electronic circuit before deposition of the components, edges of a casing to be closed, or else casing bottom) and to which a certain quantity of heat is supplied, making it possible to melt the metallic alloy. This heat transfer is most often carried out in a continuous oven.
The role of the fluxing operation before soldering is then to prepare the metallic surfaces to be soldered or tinned (performing actions such as degreasing, deoxidation, decontamination of adsorbed layers or other surface preparation), this being with the aim of facilitating the subsequent wetting of these surfaces by the solder, and also to eliminate oxides which might form on the soldering alloy.
This fluxing operation is most commonly carried out using chemical fluxes which are often obtained from resin bases, supplemented in particular by acidic compounds. It is carried out, in the case of a wave soldering machine, in an upstream zone of the machine (therefore before the support to be treated comes into contact with the solder wave) or else, in the case of reflow soldering, by virtue of the fact that the chemical flux is comprised in the composition of the soldering paste, and the fluxing phase is then carried out during a first part of the thermal cycle employed.
After soldering, flux residues then remain on the article, often requiring the manufacturer to carry out a cleaning operation, most often using chlorinated solvents which are highly controversial in the context of the "Montreal Protocol" and its subsequent revisions.
Among the ways envisaged for finding a replacement solution to the use of these compounds, mention may be made of dry fluxing processes, such as plasma fluxing of the surfaces before soldering, thereby avoiding the use of chemical fluxes and therefore the actual requirement for a downstream cleaning operation. The mixtures envisaged use, in particular, hydrogen.
In this field, mention may be made of document EP-A-0,427,020, which proposes treating assembly parts to be soldered using a plasma of a process gas, recommending the use of low pressures for this treatment "with the aim of avoiding thermal damage to the assembly parts". All the examples given, in conjunction with the figures provided, relate to pressure conditions lying in the range of 30-100 Pa.
The same comment can be made in regard to document EP-A-0,371,693, which relates to a method of fluxing metallic surfaces before soldering using microwave plasma containing hydrogen. Here again, it is recommended to use low pressures "in order to make it possible to limit the level of residual oxygen in the plasma".
This consensus of opinion in favor of the use of low pressure conditions for carrying out these plasma fluxing operations, despite the drawbacks which are in particular linked with the cost of obtaining such pressures or else with the difficulty of installing the corresponding infrastructures in an industrial production line, is undoubtedly linked with the technical and technological difficulty of obtaining, at atmospheric pressure, plasmas which give performance comparable to that which is traditionally obtained at low pressure.
In this context, the Applicant Company proposed, in document FR-A-2,697,456, a method of plasma fluxing metallic surfaces before soldering, at atmospheric pressure and using, in order to create the plasma, a microwave source or else a dielectric-barrier discharge transferred via slots placed suitably in a dielectric layer placed above the article to be treated. Although this document provides an advantageous solution to the problem in question, the Applicant Company has demonstrated the fact that the proposed method might be improved, especially as regards:
Continuing its work on this subject, the Applicant Company then more recently proposed, in the French Patent Application published on Jun. 16, 1995 under number FR-A-2,713,528, an improved method for dry fluxing a metallic surface before soldering or tinning using an alloy, which is noteworthy in that it employs the following steps:
The examples developed in this document clearly demonstrate that it is possible, using this method, in particular:
The treatment atmosphere which is obtained from the primary gas mixture, itself obtained at the gas outlet of an apparatus for forming excited or unstable gas species, may moreover according to this document also comprise, if appropriate, an adjacent gas mixture which has not passed through the apparatus.
This configuration may therefore be referred to as a "post-discharge" configuration since the primary component of the treatment atmosphere, which comprises excited or unstable gas species, is obtained at the outlet of the apparatus, which ensures the substantial absence of any electrically charged species in this primary component. The adjacent component of the treatment atmosphere, which has not passed through the apparatus, is a fortiori free thereof.
This configuration furthermore makes it possible to separate clearly the site of the generation of the primary component of the atmosphere from the site at which it is used, which has a not inconsiderable advantage in terms of contamination of the apparatus (avoiding various releases resulting from the operation of fluxing the surface from contaminating the interior of the apparatus, for example its electrodes), and also in terms of reproducibility of the atmosphere encountered by the article.
Finally, the article, which is not treated within the apparatus (for example within the discharge between the electrodes), benefits from much better flexibility with regard to the "distance" aspect mentioned above.
In this document, this method was more particularly exemplified and illustrated with the aid of a particular device for forming excited or unstable gas molecules, operating substantially at atmospheric pressure, which the Applicant Company had previously developed and which is described in document FR-A-2,692,730.